Capacitors may be used at the input side of an amplifier to remove any DC offset in the amplifier input signal, thereby preventing DC current from flowing to the speakers. In the event these capacitors fail or are not present (e.g., due to an incorrect wiring) substantial damage may be caused to the amplifier and/or the speakers. Audio systems, e.g., for car audio may include a DC offset detection function based on different approaches implemented in the analog domain.
Examples of such approaches are clipping event count or detecting the time elapsed between consecutive zero crossing events of the analog signal, that is, between events leading the analog signal to enter/exit a certain amplitude window. In the latter case, a vertical displacement of the audio signal as possibly generated by a DC offset may cause the pulses indicative of zero crossing to appear, disappear, move, mix together or change length. A DC offset detection circuit sensitive to such events may thus detect an offset voltage difference between, e.g., the output(s) of an audio power amplifier and the audio processor feeding such amplifier.
In certain implementations, a zero window crossing flag may be used as a feedback signal for a circuit which emulates internally the zero crossing window and may check, e.g., if the signal at the loudspeaker outputs of the audio power amplifier crosses the zero level at the same time as at the output of the audio processor. In certain implementations aiming at reducing costs, the flags from 4/6 loudspeakers may be mixed together (e.g., OR-ed) and then sent to the analog processor. A certain amount of loop delay may be taken into account by a tunable external capacitor.
Such implementations rely on dedicated analog components which may be fairly bulky and expensive. Also, the amount of DC offset cannot be detected and no other possible source of impairment taken into account. The detection process may be affected by intrinsic inaccuracy due to the uncertainty of parameters (e.g., the threshold levels, gain, . . . ).